There is considerable interest in photo-induced second harmonic generation for use in light sources. The phenomenon of second harmonic generation, which essentially doubles the frequency .omega. of an input beam, is useful in a frequency doubled light source (2.omega.). Higher frequency light not only can carry more information but also can be used to store and read information in correspondingly higher densities.
Photo-induced second harmonic generation has been observed in glass optical waveguides doped with certain materials, including germanium-doped fibers and erbium doped fibers. The fiber is initially conditioned by simultaneous exposure to a fundamental beam at .omega. and a second harmonic beam at 2.omega.. This conditioning process induces a permanent periodic modification of the glass resulting in an .chi..sup.(2) grating that is phase matched for second harmonic generation of light at 2.omega.. After conditioning, input of light at .omega. results in output at both .omega. and 2.omega..
One shortcoming of photo-induced second harmonic generators is their relatively low conversion efficiencies. Reproducible efficiencies have been about 10.sup.-2 % per watt or less, and higher efficiencies are desired.